The present invention relates to apparatus for the gas coating of particles suspended in a fluidized bed. More particularly, the invention relates to such apparatus for the gas coating of particles such as nuclear fuel particles under high temperature conditions while facilitating gravity unloading of coated particles therefrom.
It is well known in the prior art to employ coatings of pyrolytic carbon or metallic carbides, for example to provide protection for nuclear fuel particles used in nuclear reactors. Such particles are normally small, for example on the order to 500 microns, and may be formed from a suitable fissile material such as uranium, plutonium, thorium, or a suitable compound thereof.
Within a nuclear reactor, the nuclear fuel particles are exposed to conditions of high temperature and severe irradiation over long periods of operation. In order to assure continued effectiveness within such an environment, the fuel particles are commonly coated with an impermeable material which retains gaseous and metallic fission products within the confines of the individual particles. Such coatings may be developed through high temperature decomposition of a suitable hydrocarbon such as acetylene, propylene, propane or methane for example.
Examples of fuel particles provided with such coatings are disclosed and set forth for example in U.S. Pat. No. 3,325,363, issued June 13, 1967 to Goeddel et al.; U.S. Pat. No. 3,298,921, issued Jan. 17, 1968 to Bokros et al.; U.S. Pat. No. 3,361,638, issued Jan. 2, 1968 to Bokros et al.; and U.S. Pat. No. 3,639,452, issued Mar. 14, 1972 to Chin et al.
A preferred method for coating nuclear fuel particles with a suitable material comprises the deposition of the coating material through the high temperature decomposition for example of gaseous hydrocarbons such as those listed above. Preferably, the coating operation is efficiently carried out with the particles being suspended in the form of a fluidized bed within a high temperature coating chamber. Levitation or suspension of the particles within the fluidized bed is commonly achieved through the controlled introduction of a hydrocarbon gas, an inert carrier gas or a combination thereof beneath the particle bed. Most commonly, an inert carrier gas such as argon, helium, nitrogen or hydrogen is employed for this purpose.
Within a preferred configuration for such a coating chamber, the coating chamber base is preferably in the form of an inverted conical member which is porous or otherwise provided with means for introducing the levitating gas beneath the particle bed.
Within the coating chamber, the small nuclear fuel particles tend to be suspended within the fluid bed under generally isothermal conditions. The reactant gas is introduced into the high temperature environment of the coating chamber and decomposed to provide the coating material for the particles. The various conditions for carrying out such coating operations are well known, including temperature ranges within the coating chamber as well as the rates of pressures under which both the reactant and levitating gases are introduced into the chamber and the duration of the coating operation.
Such high temperature gas coating operations, employing fluidized beds of particles and carried out in accordance with the prior art, have encountered numerous problems. Of particular concern within the present invention are the problems of efficiency for the coating operation and design simplicity for the coating apparatus.
In connection with efficiency of operation, a substantial delay has been noted in the past since the particles are coated in batches within the coating chamber and must be removed from the high temperature chamber before a subsequent coating operation may be commenced with a new batch of particles. Two common techniques for unloading the loading chamber include vacuum removal of the coated particles through a vacuum probe and the forming of an unloading port in a lower portion of the chamber to permit gravity flow of the coated particles from the chamber.
Vacuum removal of the particles is generally undesirable since the particles must first be substantially cooled in order to prevent their being damaged during unloading. Gravity unloading of the particles has been a problem in the past because of difficulties in designing the coating chamber to provide an unloading port and also because of a tendency for carbonaceous deposits to interfere with gravity flow of particles from the chamber.
At the same time, the construction of such coating chambers has been relatively complex in the past because of the need for supplying both levitating and reactant gases to the coating chamber while preventing decomposition of the reactant gas before it is intermixed with the particles. The construction of the coating apparatus has been further complicated by the need for providing rapid and efficient unloading means as described above.
Accordingly, there has been found to remain a substantial need for improved apparatus for the gas coating of particles suspended in a fluidized bed permitting efficient removal of the coated particles from the apparatus.